Decorative graphics for automotive trim and ornamentation have been conventionally produced by screen printing an ink onto an adhesive coated film. Image-enhancing particles are often incorporated into these printing inks to provide an interesting visual appearance such as sparkle, color flop, iridescence or luster. Representative examples of image-enhancing particles include metallic flake and spherical particles, such as aluminum flake or aluminum spheres, pearlescent flake pigments such as metallic oxide coated mica, metallic oxide coated glass flake, and metallic oxide coated polyester flake. These image-enhancing particles are usually in the 1-200 microns diameter size range. Particles in the range of about 1-20 microns generally exhibit more of a lustrous appearance, while particles larger than 20 microns generally have an increasing amount of a sparkle appearance, that increases as the particle size increases. Some image-enhancing particles are more functional in nature. For example, phosphors can be used to make an electroluminescent lamp or metallic coated glass beads can be used to provide retroreflection.
However, it would be desirable to replace analog printing methods such as screen printing with a digital printing method in order to reduce cycle times and produce short runs economically. In addition, most digital printing processes eliminate the need for printing plates and significantly reduce job set-up and changeover times.
While digital color printing is well known in the graphics industry, digitally printing the breadth of image-enhancing particles used in the screen printing industry has largely been ignored. This may be due to the particle size and/or the conductivity of many image-enhancing particles such as, for example, aluminum flake. The use of a particular type of image-enhancing particle, titanium oxide coated flake-form inorganic crystal, in colored toner formulations is taught in Japanese Patent Kokai No. Sho 621987!-100771. Kokai No. Hei 11989!-112254 further teaches use of the above mentioned flake-form particles first coated with a black titanium oxide layer in toner formulations that are preferably colored. However, a wider range of decorative or functional effects are desired requiring a much wider range of image-enhancing particles.
Known methods of utilizing image-enhancing particles in solid toners involve compounding a separate batch of toner containing image-enhancing particles for each color in which an image-enhancing effect is desired. For example, green toner may be compounded with metallic flake to produce a metallic green color. Likewise, if a metallic red was desired, metallic flakes would be compounded with red toner, etc. Thus, for every different color and concentration of image-enhancing particles, a separate batch of toner compounded with image-enhancing particle was required. Making small batches of toner and image-enhancing particles is a costly process with no economies of scale. Therefore, it would be further desirable to achieve multiple color image-enhancing effects without having to produce multiple batches of color toner containing image-enhancing particles.
Because print resolution is largely determined by the particle size of toner and many desirable image-enhancing effects require particle sizes in excess of conventional higher resolution toner particles sizes, it would be still further desirable to print digitally larger image-enhancing particles without sacrificing overall image resolution.